Bo Guagua, who was studying in the US when the scandal unfolded, has since kept a low profile.

"It has been 18 months since I have been denied contact with either my father or my mother. I can only surmise the conditions of their clandestine detention and the adversity they each endure in solitude," he said in a statement to the New York Times.

"I hope that in my father's upcoming trial, he is granted the opportunity to answer his critics and defend himself without constraints of any kind."

"However, if my well-being has been bartered for my father's acquiescence or my mother's further co-operation, then the verdict will clearly carry no moral weight."

Some reports have suggested that Ms Gu may testify against her husband at the trial. The BBC's Martin Patience reports that there has been speculation that both the parents agreed to demands from Chinese officials in return for a guarantee that their son would not be pursued.

Bo Guagua also spoke out in defence of his mother, describing her as "silenced and defenceless" and voicing concern about her state of health.

"She has already overcome unimaginable tribulation after the sudden collapse of her physical health in 2006 and subsequent seclusion," he said.

State media, in reports of Gu Kailai's trial, said she attributed her actions to a breakdown.

Bo Guagua has been in the US since the investigation into his parents began

Bo Guagua last released a statement in September, saying he found the allegations against his father hard to believe.

He is soon to begin studying law in New York, he confirmed to the New York Times.

The scandal emerged after Bo Xilai's police chief, Wang Lijun, fled to the US consulate in February 2012.

A month later, Mr Bo was removed from his post as party chief in Chongqing and then stripped of his Communist Party posts as his wife was investigated for the murder of Mr Heywood.

Gu Kailai was subsequently convicted of poisoning the businessman to death and is in prison, as is the police chief, Mr Wang, for abuse of power related to the subsequent cover-up.

Bo Xilai, meanwhile, has not been seen in public since March 2012.

He goes on trial on Thursday in the city of Jinan accused of taking advantage of his office to accept money and property, as well as embezzling public money.

Most political analysts believe that the outcome of the trial has already been decided and that Mr Bo will almost certainly be found guilty, our correspondent reports from Beijing.